A composite film comprising a metallic foil with a polyimide layer laminated thereon is useful for an electric circuit board. Such composite films have been produced by the following procedures: (A) a method comprising bonding a polyimide film to a metallic foil through an adhesive; (B) a method comprising heat-bonding a polyimide film to a metallic foil; and (C) a method comprising applying a solution of a polyimide precursor in an organic polar solvent onto a metallic foil, drying the coating and imidating the polyimide precursor to form a polyimide layer.
The method (C) has several advantages as compared to the methods (A) and (B), such as that the process can be simplified because it is not necessary to form a film in advance as in the methods (A) and (B), a thin composite film can be produced, and the process is free from troubles due to the use of an adhesive as in the method (A). In the method (C), however, the coating shrinks when cooled after heating for drying or imidating, and curling occurs in composite films thus produced because it is difficult for the metallic foil to conform to the shrinkage of the coating. Thus, the composite films of method (C) have a disadvantage in that they cannot be used in the preparation of electric circuits, or that they are not convenient to use for such preparation.